In the field of aviation, it is expected that a system for controlling a piece of equipment will be available continuously. This means that in the event of a failure of any component in the system, the control system must be capable of operating, e.g. in a “limp-home” mode, so as to operate the equipment in spite of the failure. Thus, when a failure is detected in flight, the system can continue to operate.
In this context, the control systems actually in use in the field of aviation are hydraulic: they comprise a hydraulic actuator of the cylinder type, and a hydraulic network interposed between the cylinder and a control member.
The network then has two hydraulic circuits that are distinct and independent: in the event of either circuit failing, e.g. as a result of a leak, the other circuit remains available to control the hydraulic actuator.
Present trends in aviation are leading to hydraulic control systems being replaced by electrical control systems.
In this context, it has been decided to use actuators having motors that are electrical machines, in particular of the synchronous type having permanent magnets, which machines can be servo-controlled in speed, in position, or in force, while also being sufficiently lightweight.
Varying the speed of such machines requires the frequency of the current or the voltage that is applied thereto to be varied, which requires a static converter to be incorporated in order to vary the control in compliance with control variables. When the electrical power supply is single-phase, three-phase, or polyphase, the converter may be constituted by a rectifier that is not controlled, having diodes, or by a rectifier that is controlled, which rectifier is associated with an inverter.
Nevertheless, that type of inverter has arms, each of which includes two control switches or transistors that may become blocked in an open state or in a closed state. When one of the transistors is faulty, control is significantly complicated by the fact that the magnitude of the current passing through the arms that remain sound can then no longer be controlled.
Various solutions can be envisaged for the electrical power supply: the electrical power supply network may be a single three-phase network that is sufficiently secure to be considered as being reliable.
It is also possible for two distinct three-phase networks to be provided, in a manner analogous to hydraulic control systems. The two networks may then be either electrically isolated from each other or they need not be isolated.
When the two three-phase networks are isolated, the neutral of one of them is independent of the neutral of the other, and when it is not possible for them to be electrically isolated, their neutrals are either at the same potential or else they are connected together via inductive and/or resistive elements.